The present invention relates to a process for the formation of miniaturized deposits of getter materials, namely, deposits having lateral dimensions of less than one millimeter, and generally from a few micrometers to hundreds of micrometers. The invention also relates to deposits of getter materials so obtained.
The getter materials have the characteristic of being able to fix gaseous traces, such as hydrogen, oxygen, carbon oxides, water vapor and, in some cases, nitrogen. These materials are generally metals belonging to the III, IV and V transition groups (groups of the scandium, titanium and vanadium metals) or alloys thereof with other elements, generally transition metals or aluminum. The most widely employed getter materials are titanium-based alloys and, in particular, zirconium-based alloys.
A recent field of use of getter materials is represented by the micromechanical devices, generally known in the field as “MicroElectroMechanical Systems” or “MicroOptoElectroMechanical Systems,” and with the abbreviations MEMS and MOEMS (in the following reference to MEMS will only be made, also meaning the MOEMS). These devices comprise a sealed cavity inside which a micromechanical part able to perform a predefined movement or parts able to interact with an electromagnetic radiation are present, in addition to auxiliary parts and electrical feedthroughs both for supplying the device and for the transmission of signals to the outside. Examples of these devices are the microaccelerometers, described in numerous patents, such as U.S. Pat. Nos. 5,594,170; 5,656,778 and 5,952,572; the miniaturized resonators, used in the telecommunications field and particularly in the manufacturing of mobile phones, described in U.S. Pat. Nos. 5,821,836 and 6,058,027; or the miniaturized IR sensors, an example of which is described in U.S. Pat. No. 5,895,233.
At the end of the manufacturing process, several gases are contained in the cavity of a MEMS (being residual of the process or due to the degassing of the cavity walls themselves), which may interfere with the MEMS operation. For example, they can alter the movement of the movable micromechanical parts (by modifying the viscosity of the medium in which the part is moving) or modify the thermal conduction in the system, thus altering the temperature measurement in the case of an IR sensor.
It is thereby necessary to introduce into the cavity a getter material, capable of removing these gases. The use of getter materials in MEMS devices is described, for example, in U.S. Pat. Nos. 5,952,572; 6,499,354; 6,590,850; 6,621,134; and 6,635,509 and in U.S. patent application publication U.S. 2003/0138656 A1.
In the last generation MEMS the cavity has extremely reduced dimensions, and the getter can be inserted only in the form of a thin layer, being of lateral dimensions between hundreds of micrometers (μm) and a few millimeters, and with thicknesses varying between fractions of μm and a few μm. In addition, the MEMS are manufactured with technologies derived from those of semiconductors, where thousands of miniaturized devices are simultaneously manufactured on a single support (commonly a silicon wafer), by means of localized deposits and selective removals of layers of different materials. For these productions, it is necessary to be able to grant both dimensional and positioning precision of the various layers deposited, and this also applies to the deposits of getter materials.
A technique allowing the production of thin deposits with a high precision of dimensions and of deposit positioning is the one known as “lift-off,” which consists in forming a layer of photohardening polymeric material on a support (these materials are known in the field as “resist”); selectively exposing the polymeric layer by means of a mask, generally to UV radiations; selectively removing with a first solvent the previously unexposed part (or the exposed one, according to the resist and the solvent type); depositing on the support and on the resist not removed by the first solvent a thin layer of the desired material, e.g., a metal or an oxide; and finally removing, with a second solvent, the resist previously polymerized by light, thus leaving on the support deposits of the desired material only in the proximity of the apertures formed by the first solvent on the resist layer. As a deposition technique, evaporation is nearly exclusively used in processes of the lift-off type, as set forth, for example, in European patent application publication EP 341,843 and International patent application publication WO 03/043062. This technique is, however, poorly suitable for the deposition of getter material layers, because the deposited layer becomes compact and thereby without the characteristics of great surface and porosity necessary for obtaining the getter functionality.
For the production of getter material layers, it is preferable to use the cathodic deposition technique, commonly referred to as “sputtering.” In this technique, the support on which forming the thin layer is desired and a “target” of the material desired to be deposited are arranged in a process chamber; the chamber is first evacuated and subsequently filled with a noble gas atmosphere, commonly argon or krypton, at a pressure generally comprised between about 0.01 and 0.1 Pascal (Pa); by applying a potential difference of some thousands of volts between the support holder and the target holders (so that the latter is at the cathodic potential), a plasma of ions (Ar+ or Kr+) is created, which are accelerated by the electric field toward the target, causing impact erosion thereof; the species (generally atoms or clusters of atoms) resulting from the erosion of the target deposit on the support, thus forming the thin layer. With a proper definition of the process parameters, this technique can be suitable for the formation of getter material layers.
However, as it is well known in the field of deposition of thin layers, the use of sputtering in lift-off processes is troublesome.
A first problem occurring is that, during sputtering, an overheating of the resist and consequent hardening thereof take place, whereby the resist layer can no longer be removed with solvents; the problem is well known in the field, and is described, e.g., in the article “Low-noise MESFET with sputtered amorphous metal gate defined by lift-off,” by N. A. Papanicolaou et al., Inst. Phys. Conf. Ser. No. 65, pags 407-414 (see pag. 411 in particular). In order to overcome the problem, this article suggests cooling the support during the deposition at a temperature of about 10° C. However, in addition to rendering the apparatus complex, this has the consequence of reducing the density of the deposited layer, which is an effect not generally desired in productions where lift-off is adopted.
The second problem of the use of sputtering is that in this technique the material deposition is not directional, i.e., the material deposits on the support in all directions rather than in a preferential direction (as happens, in contrast, with evaporation). This characteristic causes the target material to deposit uniformly on all the available surfaces, forming a continuous layer on the upper surface of the resist, at the bottom of the cavities formed in the resist (i.e., on the support exposed zones) and on the lateral walls, made of resist, of these cavities. The continuity of the deposited layer prevents the subsequent access of the second solvent to the resist and hence the removal thereof from the support surface. The problem is tackled in numerous documents of the prior art, offering various solutions which, however, always require the use of particular expedients.
A first expedient is to provide that a recess (known in the field as “undercut” or “notch”) is present under the resist layer along the whole periphery of the cavity, sufficiently deep to be only barely filled with the material being deposited; in this way the continuity of the deposited layer is interrupted, thus leaving a way of access to the solvent in order to reach the contact zone between the resist and the support surface. However, the formation of the recess generally requires that the resist layer actually be a double layer of different polymeric materials, with different solubility characteristics in different solvents, so that the lower layer (the one in direct contact with the support) is attacked by the first solvent more rapidly than the upper one; this approach is illustrated, for example, in U.S. Pat. No. 5,705,432 and in patent application publications EP 341,843 and WO 03/043062. The article “Introduction of complete sputtering metallization in conjunction with CO2 snow lift-off for high volume GaAs manufacturing” by F. Radulescu et al., article 11a of the Proceedings of 2002 GaAs Mantech Conference, suggests, in addition to the use of a double layer resist, a treatment after the deposition by sputtering with CO2 “snow” causing a thermal differential dilation between the resist and the deposited material on the upper surface thereof, in order to detach the deposit on the resist and to expose the latter to the solvent attack. Finally, U.S. Pat. No. 5,658,469 suggests for the formation of the undercut a sequence of irradiations of the resist with electron beams of different powers in order to make the upper part of the resist less soluble in a solvent with respect to the lower one, so that afterwards the latter can be preferentially removed.
In order to improve the sputtering directivity, it is also possible to move the support away from the target and interpose between the two parts a collimator, that is a mechanical filter, intercepting the particles moving in directions non-perpendicular (or nearly so) to the support, but these measures reduce the amount of material deposited on the support with respect to the one removed from the target, leading to waste of material, to the need for replacing the target more frequently, and, in short, an increase in process costs.
Finally, as set forth in Japanese patent application publication JP 2002-043248, in order to use in lift-off processes the deposition by sputtering, it is required that the latter occur at very low pressures, for example in the range of 0.1 Pa; this causes an increase in the energy of the “sputtered” atoms and a consequent increase in the temperature of the layer being deposited, as well as of the material laying under the deposit, with a double negative effect on the process. On the one hand, thermohardening of the resist occurs, which is afterwards more difficult (or impossible) to remove with solvents and, on the other hand, the getter material layer tends to grow too compact, and thereby without the necessary morphologic characteristics.
As a consequence of these process complications, the sputtering deposition technique does not have practical industrial application as the deposition operation in lift-off processes.